Pyramidal structure, silyl radical

Undoubtedly, the uniform structural and spectral behavior of the trigonal-planar jt-radicals (t-Bu2MeSi)E (E = Si, Ge, Sn) 43-45 both in the solid state and in solution should be ascribed to the immediate impact of the bulky electropositive silyl substituents. In contrast, it is well known that simple alkyl and aryl substituents cause a highly pronounced pyramidalization at the radical centers, where the unpaired electron typically occupies the orbital with a high s-contribution (o-radicals)7 ... 

Hybrids of the type sp3 are unjustified for disilane. An important conclusion from the above hybridization statement No. 4 is concerned with the contrasting structures of the radicals SiH3 and CH3. The planar geometry of the methyl radical can readily be explained by the (bond-strengthening) sp2-hy-bridization, while the pyramidal silyl radical is thought to be stabilized (with respect to the planar arrangement) through the s-admixture to the lone electron orbital. 

The pyramidal structure of the silyl radical provides the key to understanding that silicon is unlikely to appear in a trigonal planar form in silicon hydrides. 

All structural information discussed now favours pyramidal silyl radicals. The XSiX angle appears to be nearly tetrahedral or larger, evidence being obtained from esr hyperfine splitting. Table 24 presents the symmetry and selection rules for vibrational spectra for silyl radicals of different composition and different geometry. 

These experiments offer good chemical evidence in support of the pyramidal structure of the silyl radical and its relatively slow rate of inversion. However a marked destabilization was observed for the chiral disilanyl radical 6 (eq. [3]) (19). 

The low stereoselectivity here is consistent with ESR data, which indicated a more planar structure for the pentamethyldisilanyl radical Me3Si-SiMe286. Nevertheless, the pyramidal silyl radical exhibits significant configurational stability. The inversion of the pyramidal species, is generally slow enough to allow reactions in which the configuration at silicon is retained. 

In the silsesquioxanes, it is oxygen that couples the two radical functions, as shown schematically in the model compound 9. Quantum chemical calculations on 4 and 9 substantiate the qualitative assertions (Table 30.1). The singlet state of the C2 conformation is substantially lower in energy (117.7 kJ/mol) than the planar structure (C2V geometry), in agreement with the known strong pyramidalization forces in silyl radicals. 

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